This type of method can be used equally to measure turbulence rates (more or less strong in a given environment) and to measure the effects of turbulence on the propagation of a laser beam.
In effect, the analysis of turbulence has many potential applications, via compact and discrete means, notably in the field of the environment, of detection of temperature rises, of air movements in a town for example.
It can also be highly advantageous to measure the modifications brought about on the propagation of a laser beam by ambient turbulences. The propagation of laser beams through a turbulence medium is notably of great importance in applications such as optical communication in free space (sea, battlefield, etc.), the detection of targets, imaging and long-distance detection. In many of these applications, lasers with their electromagnetic field coherence properties are used.
The coherent fields which are propagated in random mediums such as atmospheric turbulence are subject to random spatial and temporal distortions and flickerings of the optical intensities which can lead to a considerable degradation in the performance of the system. These flickerings due to rapid and continuous changes of temperatures and of pressure close to the surface of the ground are more severe and more pronounced there and induce distortions of the phase structure of the wave field by the turbulence causing the wave to interfere with itself.
These changes in the data and parameters of the beam introduce fundamental limitations into the development of the optical communication systems in free space. One of the significant effects of atmospheric turbulence is the fluctuation of the direction of the propagation of the wave front referred to as the fluctuation of the angle of arrival or the tilt of the wave front. This angle is measured by different methods.
One of the methods consists in simultaneously performing differential measurements of the angles of arrival and of the fluctuations of intensity of the laser beam from a reference by using a CCD camera since the measurements of spatial fluctuations are highly sensitive to the wavelength of the path traveled by the laser beam (A. Consortini et al., Optics Communications, 216, 19-23, (2003)). This fluctuation depends cubically on the length of the optical path traveled.
Other methods consist in using a structuring of the laser beam either with interference fringes and by analyzing their visibility in the case of a type of Doppler laser anemometry (H. J. Pjeider et al., J. Opt. Soc. Am., Vol. 70(2), 167-170, (1980)) or else by using the displacement of the Moiré fringes (S. Rasouli et al., Opt. Lett., 31(22), 3276-3278, (2006)) resulting from the superposition, with the same step, of an image of reference fringes and an image of fringes resulting from the turbulence. The Moiré structures give the fluctuations of the angle of arrival.
In both cases, the methods are difficult to implement since the interferences have to be created for the Doppler anemometry or else two amplitude gratings have to be installed and their initial stability has to be ensured for the Moiré technique. The tilt of the wave front is, for its part, analyzed by using polynomial techniques based on an analysis method of shark-Hartmann type (A. V. Sergryev et al., Appl. Opt., 50(20), 3519-3528, (2011)) and requires a system for analyzing the wave front of the laser that is complex and costly. The fluctuations of the angle of arrival or the tilt of the wave front are used to measure the characteristic parameters of the atmospheric turbulence. The structure parameter of the refraction index or Cn2 is a measurement of the force of the optical turbulence along the laser beam propagation path. In the laser propagation in free space, the measurement of the fluctuations of the angle of arrival is a basic step in the study of atmospheric turbulence.